Submerging a print in water tests more than the ink's water resistance. Some manufacturers try to cut corners by leaving out the top resin coat. While you don't need the top coat to protect the image from water, it is important because it adds structure to the photo paper. Without it, prints will curl when they're exposed to moisture. Obviously, that's not good if you live in a humid environment.
This isn't just a problem with third-party printer paper. We also printed on Kodak's Premium Photo Paper and we found out the paper curls. Incidentally, that's how we discovered this photo paper lacks top and bottom resin coats. When your photo paper is resin coated, you should see nearly no buckling, waviness, or curling.
There are several photo papers that can be considered faux resin-coated because they lack distinctive plastic layers. Many of Kodak's photo papers fall into this category, and as a result, water resistance tends to be poor.
While immersing a photo into a tub of water shows the worst-case scenario, tearing a corner of your photo paper is the only surefire way to check for resin-coated layers. Faux resin-coated paper imitates the quality of the real thing through special surface coats. But beneath that, it's all paper fibers. With true resin-coated paper, you see the plastic threads stretch when the paper is torn.